U.S. Pat. No. 3,370,112 to Wray and U.S. Pat. No. 3,086,242 to Cook et al disclose processes and apparatus for expanding tubing by employing a pressure differential between the inside and outside of the tubing. Both of these patents are incorporated by reference herein for all purposes. Cook et al and Wray describe processes for producing heat recoverable tubing by cross-linking and expanding a tube of polymeric material. In both cases, the tubing is expanded by employing a pressure differential between the inside and outside of the tubing. The first step in the process is to cross-link the tubing. After the tubing is cross-linked, it is heated to a temperature equal to or above its crystalline melting temperature or range so as to melt the crystals in the material. While the tubing is at the elevated temperature, a pressure differential is imparted across the tubing wall to expand the tubing. While subjected to this differential pressure, the tubing is passed through a cooling zone to cool the tubing to a temperature below the crystalline melting temperature or range. The pressure differential is imparted by continuously supplying air to the interior of the tubing, while applying ambient or sub-ambient pressure outside the tubing. Upon re-heating, the tubing will recover to the configuration it had when cross-linked.
An alternative method of imparting a pressure differential is to trap a bubble of air within the polymeric tubing while again applying ambient or sub-ambient pressure outside the tubing. Over time, the pressure in the tubing bubble will drop as a result of permeation of gas into the tubing wall and also by way of air escaping through the ends of the bubble of air. As the pressure drops, the outer diameter of the tubing becomes smaller, resulting in production of tubing that may not meet the required specifications.
An attempt to increase the amount of tubing processed which meet desired specifications by using a bubble of air is described in U.S. Pat. No. 3,687,585 to Takagi et al. Takagi et al discloses confined movement between the ends of a bubble of air. However, the increase in the amount of tubing processed is limited by the space available in which the ends of the bubble of air can be linearly separated from each other. Once the space has been consumed, additional air must be supplied to the inside of the tubing by ceasing production and restarting the process.